A device for detecting an abnormality in an air/fuel ratio sensor (hereinafter also referred to as a “A/F sensor”) by utilizing an output from the A/F sensor, for example, as disclosed in Japanese Patent Laid-Open No. 2005-30358, is known. According to this conventional art, an injection quantity, i.e. an amount of fuel from an injector, is changed by open-loop control and determination as to the existence/nonexistence of an abnormality in the A/F sensor is made by evaluating changes in the output from the A/F sensor resulting from changing the amount of duel injection.
Also, an air/fuel ratio control apparatus for internal combustion engines is known in which an output from an A/F sensor is fed back to an injection quantity, for example, as mentioned in Japanese Patent Laid-Open No. 2004-68602. According to this conventional art, a plant model representing a control system from a fuel injection valve to the A/F sensor is identified at successive times, and feedback control is performed by using the values of parameters identified in this model.    [Patent document 1] Japanese Patent Laid-Open No. 2005-30358    [Patent document 2] Japanese Patent Laid-Open No. 2004-68602    [Patent document 3] Japanese Patent Laid-Open No. 5-180059    [Patent document 4] Japanese Patent Laid-Open No. 8-232727    [Patent document 5] Japanese Patent Laid-Open No. 2006-118429    [Patent document 6] Japanese Patent Laid-Open No. 2003-97334    [Patent document 7] Japanese Patent Laid-Open No. 2004-316457    [Patent document 8] Japanese Patent Laid-Open No. 2004-360591